1. Field of the Invention
This invention relates to a photomask and its method of manufacture, in particular, this invention relates to a substrate engraving type phase shift mask and its method of manufacture.
2. Related Arts
The progress of optical lithography technology for forming a more detailed resist pattern on a wafer is remarkable. In such a field, the phase shift method has been adopted in which pattern resolution can be improved by mutually varying phases of light that penetrate two adjacent transparent portions on a mask.
This method carries out projection exposure on a wafer by using, a shifter for reversing the phase from one of the adjoining translucent parts, in which a mask is formed according to the relation d=ë/2 (n−1), wherein d is the film thickness, n is the refractive index, and ë is the exposure wavelength (hereinafter, the mask is called the “phase shift mask”.) Since the light which passes through the shifter has a reverse phase (180 degrees) to the light transmitted through the adjoining translucent part, the optical intensity is set to 0 in the pattern boundary part, and the pattern can be separated, to improve resolution.
A mask shaped to realize the high resolution of the above-mentioned phase shifting method is described. There is a shifter formation type phase shift mask, as shown in FIG. 6A, which has a transparent medium (also called “a phase shift film” or “shifter”) 530 at one of the two adjacent openings (translucent parts) 521 having a refractive index differing from air. In the case of this mask, however, it is difficult to deposit the phase shift film with sufficient accuracy because it has the same refractive index as the substrate, and because of multiplex reflections at the phase shift film 530.
Various substrate engraving type phase shift masks (they are also called “engraving type phase shift masks.”) which dig the transparent substrate by etching, etc., which solve these problems are known. The substrate engraving type phase shift mask shown in FIG. 6C and FIG. 6D is in use especially now.
The engraving type phase shift mask shown in FIG. 6C is what is called a “single-side engraving type phase shift mask”, which is manufactured, as shown in FIG. 6B, by dry-etching a substrate (Qz is generally used) partway to create a recess, and additionally, wet etching only the dug-down part (recess) so as to obtain the predetermined phase difference (usually 180 degrees). The engraving type phase shift mask shown in FIG. 6D is what is called a double side engraving type phase shift mask”, which is manufactured, as shown in FIG. 6B, by dry-etching a substrate to obtain the predetermined phase difference (usually 180 degrees), and additionally, wet etching the whole surface.
In the substrate engraving type phase shift mask (having a predetermined phase difference (usually 180 degrees) by FIG. 6B), which is manufactured by subjecting the substrate to perpendicular dry etching simply, there is a problem that the degree of the penetration light intensity is varied by the existence of a substrate engraving in the opening. The substrate engraving type phase shift masks shown in FIG. 6C and FIG. 6D have been developed to solve this problem.
Generally, what is called the “substrate engraving type phase shift mask” refers to the phase shift mask containing the pattern from which the shading part, which shades exposure light, and the translucent part are formed on the transparent substrate having the transparent refractive index n to the exposure light of wavelength ë, and wherein mutually adjacent translucent parts are formed by digging down the transparent substrate at the translucent parts so as to satisfy the condition that d1−d2 is almost equal to ë/2 (n−1), wherein d1 is the depth of one recess at one side, and d2 is the depth of another recess adjacent to the former recess.
Hereinafter, the case where either one of d1 or d2 is 0 will be referred to as a “single-side engraving type phase shift mask”, and the case where both d1 and d2 are 0 will be referred to as a “double-side engraving type phase shift mask”.
Further, hereinafter, the recess having the larger recess depth (also called the “dug-down amount”.) will be referred to as the “shifter part”, and the other recess having the smaller recess depth will be the “non-shifter part”.
The Levenson type phase shift mask can be mentioned as a substrate engraving type phase shift mask.
As the substrate engraving type phase shift mask, it is applicable to use a phase shift mask for KrF excimer lasers whose exposure light has a wavelength of 248 nm, a phase shift mask for ArF excimer lasers whose exposure light has a wavelength of 193 nm, and a phase shift mask for F2 excimer lasers whose exposure light has a wavelength of 157 nm.
However, when the structure of the engraving type phase shift mask as shown in FIG. 6C or FIG. 6D is adopted, the eaves 525 of shading film will be formed by wet etching, and it becomes easy to generate a chip and peeling of a shading film.
Particularly, after wet etching, the washing tolerance falls remarkably and it is impossible to perform the usual washing.
Moreover, when microstructuring of the circuit pattern progresses along with the improvement in the speed and high-densification of a semiconductor integrated circuit, peeling of the shading film occurs during wet etching in this structure, and it can be expected that manufacturing becomes difficult. Recently, in order to solve the problem on this structure, in the substrate engraving type phase shift mask on which the part to be etched (shifter part) and the part not to be etched (non-shifter part) adjoin mutually, it is proposed that there be a substrate engraving type phase shift mask, as shown in FIG. 7, having a structure where the shading film continuously covers from the end of the shifter part to the adjacent end of the non-shifter part, and the side wall 716 of the dug-down part intersects almost perpendicularly in the direction of the substrate.
The mask of the structure shown in FIG. 7, has no eaves of a shading film, so neither a chip nor peeling occurs, but a mask that has excellent durability can be produced.
However, if the substrate engraving type phase shift mask having the structure as shown in FIG. 7 is produced, it is also necessary to form a uniform shading film in the step-difference parts of a substrate.
If the shading film is formed unevenly, influence will arise for the processing accuracy at the time of plate-making, or various problems, such as a fall of washing tolerance, will occur in the thin portion of a shading film.